Question: What is the greatest common factor of $15$ and $20$ ? Another way to say this is: $\operatorname{gcf}(15, 20) = {?}$
Solution: The greatest common factor is the largest number that is a factor of both $15$ and $20$ The factors of $15$ are $1$ $3$ $5$ , and $15$ The factors of $20$ are $1$ $2$ $4$ $5$ $10$ , and $20$ Thus, the greatest common factor of $15$ and $20$ is $5$. $\operatorname{gcf}(15, 20) = 5$